1. Field of the Invention
The present invention relates to water jet pumps (hereinafter also simply referred to as jet pumps). More specifically, the present invention relates to a jet pump used as a propelling device for a ship and, in particular, to anti-cavitation technology.
2. Description of Background Art
In the background art, a jet pump is known to include an impeller, which is driven for rotation and is provided in a cylindrical housing (see, e.g., Japanese Laid-Open Patent Publication No. 2003-89394 (Abstract, FIG. 3) and Japanese Laid-Open Patent Publication No. 2002-87385 (Abstract, FIGS. 1 to 4)). With the impeller being driven for rotation, a jet stream of water occurs to propel a ship.
In a jet pump of this type, it is desired to improve the pumping efficiency. Accordingly, in a jet pump according to the background art, as shown in FIG. 7 of the present invention, the outer circumferential surface 1a of an impeller 1 that opposes the inner surface 2a of a housing 2 has been sharpened by a cutting process to be entirely parallel to the inner surface 2a of the housing in order to minimize the cavity C between the outer circumferential surface 1a of the impeller 1 and the inner surface 2a of the housing 2 and thereby improve the pump efficiency. Consequently, the edge portion 1e of the impeller 1 has not been provided with a radius of curvature.
In the jet pump according to the background art, the outer circumferential surface 1a of the impeller 1 that opposes the inner surface 2a of the housing 2 has been sharpened by a cutting process to be entirely parallel to the inner surface 2a of the housing 2, as shown in FIG. 7 of the present invention. Accordingly, when the impeller 1 is driven for rotation, water W at a high pressure on the rear surface side 1b of the impeller 1 dashes into the narrow cavity C between the outer circumferential surface 1a of the impeller 1 and the inner surface 2a of the housing 2 to be pushed out of the narrow cavity C toward the front surface side 1f of the impeller 1 under a lower pressure. As a result, cavitation is likely to occur due to a large pressure variation and air bubbles resulting from the cavitation enter the space behind the impeller 1, which causes the problem that the pump efficiency is resultantly lowered.
In the jet pump described in the above-mentioned Japanese Laid-Open Patent Publication No. 2002-87385, a connecting portion, which connects the rear surface side of the impeller to the front surface side thereof is provided in the outer circumferential surface of the impeller in order to reduce cavitation. However, the provision of such a connecting portion has a drawback in that it lowers the pump efficiency accordingly.